Monocrystalline silicon solar cells, polycrystalline silicon solar cells, amorphous silicon solar cells, and compound semiconductor solar cells are known examples of solar cells. In recent years, silicon solar cells, such as monocrystalline silicon solar cells or polycrystalline silicon solar cells are mainly used.
However, manufacture of silicon solar cells is costly, as it requires a high-purity silicon material, as well as a high temperature and high-vacuum environment.
Under such circumstances, dye-sensitized solar cells are attracting attention in recent years. The dye-sensitized solar cell has a simple structure and can be easily manufactured; moreover, the material thereof may be selected from various substances. Furthermore, the dye-sensitized solar cell ensures high photoelectric conversion efficiency.
Generally, dye-sensitized solar cells can be manufactured by using a simple method of injecting an electrolyte containing a substance having a reversible electrochemical redox characteristic, such as iodine, between a photoelectrode and a counter electrode, and then connecting the photoelectrode and the counter electrode.
The photoelectrode is generally formed by using the following method. First, a glass substrate having a conductive layer, such as ITO (Indium Tin Oxide) or FTO (Fluorine Tin Oxide), on its surface is coated with a paste containing titanium oxide fine particles. Then, the coated substrate is heat-treated at about 500° C., thereby producing an electrode having a titanium oxide layer. Further, the resulting electrode is immersed in an alcohol solution containing a ruthenium metal complex (dye sensitizer), such as red dye (N719) or black dye (N749), thereby allowing the ruthenium metal complex to adhere to the porous surface of the titanium oxide.
The counter electrode is formed by depositing a catalytic layer (for example, a platinum film), which has a catalytic activity with respect to the substance having an electrochemical redox characteristic, on the glass substrate, on which the conductive layer has been formed, by way of sputtering or the like.
However, in such a dye-sensitized solar cell, an increase in the area of the glass substrates serving as the photoelectrode and the counter electrode (an increase in the area of the conductive layer) increases the electric resistance of the conductive layer, thereby problematically decreasing photoelectric conversion efficiency. Moreover, dye-sensitized solar cells also have problems in that the electric resistance of the conductive layer is also increased by the heat treatment for making a titanium oxide sintered compact, and that the highly corrosive halogen family compound, such as iodine, contained in the electrolyte, corrodes the conductive layer (a durability problem).
In order to solve these problems, a method of forming the substrate of the photoelectrode from a titanium metal is attracting attention.
The use of titanium metal as the substrate of the photoelectrode makes it possible to reduce the increase in electric resistance, compared with hitherto known photoelectrodes that use a glass substrate having a conductive layer. Further, the titanium metal photoelectrode also suppresses the increase in electric resistance due to the heat treatment and gives high corrosion resistance against the highly corrosive halogen family compound, such as iodine, contained in the electrolyte. Accordingly, the use of titanium metal as the substrate of the photoelectrode will provide a constant resistance against iodine or the like for a long period. Moreover, since the titanium metal surface increases the affinity of the titanium oxide layer for the titanium substrate surface, a desirable flow of electrons can be expected.
However, because the titanium metal basically lacks light permeability, when the titanium metal is used as the substrate of the photoelectrode, the light must be incident on the counter electrode having light permeability. To emit light onto the counter electrode, the light must first permeate the electrolyte layer, which contains the catalytic layer and iodine, etc., provided on the counter electrode. Consequently, the light amount decreases by the time the light reaches the photoelectrode. This results in insufficient photoelectric conversion efficiency.
Patent Document 1, which was previously reported by the present inventors, discloses an example of application of titanium to a photoelectrode of a dye-sensitized solar cell. First, a titanium nitride is formed on a titanium surface, and the titanium is then subjected to application of a voltage greater than the spark discharge voltage in an electrolyte that has an etching property with respect to titanium, thereby forming an anatase titanium oxide film. However, since this solar cell has a drawback in that the light amount decreases by the time the light reaches the photoelectrode, there has been a demand for further increasing the photoelectric conversion efficiency of solar cells.
To solve this problem, a photoelectrode formed of a titanium mesh has been proposed. More specifically, Patent Document 2 uses a metallic grid with an opening area of 50 to 95% of the entire metal, thereby enabling light to be directly incident on the photoelectrode. Further, Patent Document 3 also discloses a structure of enabling light to be directly incident on the photoelectrode, using a titanium metal mesh having an opening area of 60% or greater.
However, although the solar cells of Patent Documents 2 and 3 made it possible to allow the light to be incident on the photoelectrode, the photoelectric conversion efficiency was still insufficient. In the solar cells of Patent Documents 2 and 3, it is necessary to ensure a large opening area to improve the light retrieval amount; accordingly, the area of the semiconductor layer, which is made of, for example, a titanium oxide modified with a dye sensitizer, decreases. This results in a decrease in photoelectric conversion efficiency.